Python中字符串的定义：由Unicode码点组成的不可变序列（Strings are immutable sequences of Unicode code points）。
Python内置函数提供了强大的字符串的使用方法，熟练掌握常见的方法，对于数据处理、面试、笔试都非常有用。
代码语言：javascript复制dir(str)
['__add__', '__class__', '__contains__', '__delattr__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__getnewargs__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__iter__',
'__le__', '__len__', '__lt__', '__mod__', '__mul__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__rmod__', '__rmul__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', 'capitalize', 'casefold', 'cent
er', 'count', 'encode', 'endswith', 'expandtabs', 'find', 'format', 'format_map', 'index', 'isalnum', 'isalpha', 'isascii', 'isdecimal', 'isdigit', 'isidentifier', 'islower', 'isnumeric', 'isprintable', 'isspace', 'istitle', 'isuppe
r', 'join', 'ljust', 'lower', 'lstrip', 'maketrans', 'partition', 'replace', 'rfind', 'rindex', 'rjust', 'rpartition', 'rsplit', 'rstrip', 'split', 'splitlines', 'startswith', 'strip', 'swapcase', 'title', 'translate', 'upper', 'zfi
ll']首先，字符串是一种序列，所以支持切片和索引操作，具体可以看这篇文章Python基础之数据类型详解。
下面是常见的内建函数用法总结：
s="hello testers !"
print(s.replace('hello','hi')) #输出hi testers !
s="hello testers !"
print(s.split()) #输出['hello', 'testers', '!']
print(s.partition('testers')) #输出('hello ', 'testers', ' !')
s=" hellotesters! "
print(s.strip()) #输出hellotesters!
print(s.rstrip()) #输出 hellotesters!
print(s.lstrip()) #输出hellotesters!
s1="hellotesters!"
s2="HELLO TESTERS!"
s3="Hello Testers!"
s4=s1="hello testers!"
print(s1.upper()) #输出HELLO TESTERS!
print(s2.lower()) #输出hello testers!
print(s3.swapcase()) #输出hELLO tESTERS!
print(s1.capitalize()) #输出Hello testers!
print(s4.title()) #输出Hello Testers!
s1="hello testers!"
print(s1.count('l')) #输出2
print(s1.find('hello')) #输出0
print(s1.rfind('testers!')) #输出6
s1="hellotesters!1234567890"
s2="hellotesters"
s3="1234567890"
s4="HELLOTESTERS"
print(s1.isalnum()) #输出False
print(s2.isalpha()) #输出True
print(s3.isdigit()) #输出True
print(s4.isupper()) #输出True
print(s2.islower()) #输出True
